NF.sub.3 gas is generally,used as the dry etching gas or the cleaning gas in the manufacture of semiconductors. Thus, the etching of silicon with an ionized reactive gas in NF.sub.3 discharge gives volatile reaction products and this is an advantage because, unlike the earlier method of etching in a fluorocarbon plasma, chances for fouling of wafer surfaces with reaction byproducts, e.g. carbon (C) and sulfur (S), are eliminated and accordingly the etching speed is expedited. For this reason, NF.sub.3 gas is used more often in these days but since this gas is highly stable at ordinary temperature, it is not decomposed in the atmosphere, thus, causing various adverse effects on living matter. Moreover, although it is not inflammable, NF.sub.3 gas is toxic with a maximum allowable concentration of 10 ppm. Therefore, disposal of the waste gas presents a major problem.
The applicant of the present invention already proposed a method for treating NF.sub.3 gas which, comprises reacting a NF.sub.3 -containing waste gas with a mass of carbon, such as charcoal, at a high temperature to convert NF.sub.3 into non-toxic CF.sub.4 and N.sub.2 gases (Japanese Patent Application No. 78883/1986) For carrying this method into practice, granular carbon as the mass of carbon is packed into a cylindrical reactor and said NF.sub.3 -containing waste gas is passed through the layer of granular carbon so as to convert NF.sub.3 to non-toxic CF.sub.4 and N.sub.2 gases in the process. This method is advantageous in that NF.sub.3 is converted to non-toxic CF.sub.4 and N.sub.2 and that even if O.sub.2 is contained in the NF.sub.3 waste gas, it is converted to CO.sub.2 gas. Another known technique for treating a NF.sub.3 -containing waste gas comprises the use of silicon as a catalyst. However, this method has the disadvantage that it requires an additional step for treatment of poisonous SiF.sub.4. Recently, O.sub.2 has been used in combination with NF.sub. 3 for enhanced cleaning effect but when this silicon catalyst method is applied in such cases, O.sub.2 reacts with silicon to give SiO.sub.2, which is a solid, and this SiO.sub.2 tends to plug the piping. In the above-mentioned method employing a mass of carbon as a catalyst as previously proposed by the present applicant, NF.sub.3 is directly converted to non-toxic CF.sub.4 and N.sub.2 gases and even if O.sub.2 is contained in the NF.sub.3 -containing waste gas, O.sub.2 reacts with carbon to give CO.sub.2 gas, thus causing no such troubles as plugged piping. However, when a production-scale plant was constructed and operated, the following disadvantage was discovered. As the grain size of granular carbon is gradually decreased as the reaction continues, the intergrain gaps are progressively narrowed to increase the flow resistance of the NF.sub.3 -containing waste gas, with the result that the pressure differential between the inlet and outlet ends of the reactor is also progressively increased. Therefore, the back pressure (pressure at the outlet end) of the semiconductor manufacturing equipment connected to reactor through a pipeline is also increased and accordingly the internal pressure of the semiconductor manufacturing equipment is also increased to interfere with stable operation. In the above apparatus, the degree of consumption of granular carbon is ascertained by detecting the top level of the carbon packing in the cylindrical reactor with a level gauge utilizing a laser beam and the reactor is refilled with granular carbon when the top level falls below a reference level. In this apparatus, the above refilling results in an increased thickness of the carbon layer and the intergrain gaps are decreased (dust carbon enters into the gaps between grains) so that the flow resistance of the NF.sub.3 -containing waste gas is sharply increased to cause a rapid elevation of the back pressure of the semiconductor manufacturing equipment, thus interfering with stable operation.